It's a straight-forward adventure style story, with a very hokey premise. The background is the deathly rivalry between Werewolves and Vampires (which I think is a recent Hollywood invention), and a secret society which hunts down supernatural beings ('World Organisation for the Control of Occult Phenomena' hmmph). The story is simple, with quite a lot of sex and violence, none of it too extreme. The protagonist is an action-hero James Bond type, staying in flashy hotels and drinking expensive alcohol.
Two aspects elevate it well above the Dan Brown basement of schlock novels. One is that Duncan has a reasonable grasp of politics, history and psychology. I don't mean it's marked by uniquely conceived insights or anything, but it's not offensively stupid like so many action stories are. For a story with quite a lot of sex, most of it with prostitutes, the portrayal of women was not offensive either. Furthermore, in stories like this the creature is sometimes sentimentalised - made to kill only people like Nazis, whom the novelist thinks deserve it for example - but I think this story manages to strike a reasonable balance, not over-sympathetic. Reading back through this paragraph it sounds like I am damning with faint praise, but to be honest I think it's a real achievement to write a populist rollicking novel which isn't as dumb as the rocks.
The other feature which elevates it above the baseline is that at a sentence level it is written in a fairly elaborate and assured style. Big vocabulary, diverse literary and philosophical references, original metaphors. Some might find it over-written or pretentious. Personally I am happy with this style. Here is a paragraph taken more or less at random to illustrate what I mean:
Reader, I ate him. About three hours after resolving that I wouldn't. Throughout the dull solo feast the refrain from Tennyson's Mariana repeated in the hot spaces of my gorging head:She only said my life is dreary, he cometh not she said
She said, I am aweary, I would that I were dead.
I guess that is something you either like or not. I like it fine. This book is first of a trilogy. I will read #2 when it comes out, and I also expect this one day to be a pretty good film franchise.